1. Field of the Invention
The present invention relates to a method of treating exhaust gases, especially those discharged from diesel engines, gasoline engines or gas turbines which contain excess oxygen and moisture as well as nitrogen oxides (abbreviated as NO.sub.x hereinafter). The method comprises bringing the exhaust gas into contact with a zeolite catalyst in the presence of organic compounds, thereby converting NO.sub.x in the exhaust gas into harmless nitrogen gas.
2. Description of the Prior Art
There are several practical methods for removing NO.sub.x from exhaust gases. For example, the selective reduction method is applied to exhaust gases from boilers which employs the V.sub.2 O.sub.5 -TiO.sub.2 catalyst and ammonia gas as the reducing agent. The method applied to exhaust gases from gasoline engines consists of controlling the air-fuel ratio (hence the oxygen concentration) and removing NO.sub.x, carbon monoxide, and hydrocarbons all at once by the use of the ternary catalyst. (See Funahiki and Yamada, "Catalysts for Automotive Exhaust Gas", Preprints of the Meeting for Theoretical Fundamental Study of Practical Catalysts, Catalysis Society of Japan, p. 15-20, 1989.) The former method has an advantage of being effective for removing NO.sub.x from exhaust gases containing excess oxygen, but it has also a disadvantage of requiring ammonia gas as the reducing agent. Therefore, it is useful for special applications but not for general uses. Especially, it can hardly be applied to vehicles carrying a diesel engine of compression ignition type and to small- or medium-sized stationary boilers. The latter method using the ternary catalyst is not effective for exhaust gases containing excess oxygen and hence it is not of practical use for exhaust gases from diesel engines.